pancake day vs Fat Tuesday

pancake day and Fat Tuesday both are nouns.

Word	Noun	Adjective	Verb	Adverb
pancake day	Yes	No	No	No
Fat Tuesday	Yes	No	No	No

As nouns, pancake day and Fat Tuesday are synonyms defined as:

Other synonyms of pancake day include Mardi Gras, Shrove Tuesday.

pancake day (noun)	Fat Tuesday (noun)
the last day before Lent	a carnival held in some countries on Shrove Tuesday (the last day before Lent) but especially in New Orleans
	the last day before Lent